The beam blanker is an integral component of electron optical columns dedicated to chip and mask lithography, circuit testing and inspection, and other applications where a charged particle beam must be rapidly turned off and on at a target plane (i.e., the surface of a substrate). A typical beam blanker 1 is shown in FIG. 1. A steady state voltage applied to beam blanker 1, comprising two identical vertically aligned U-shaped plates 2 spaced apart by distance 3, produces an electric field transverse to a beam axis 4. This field deflects the electrons or ions (which comprise the beam emanating from beam source 11) off-axis away from aperture 5 causing the electrons or ions to become embedded in beam stop 6, thereby stopping any beam current from being transmitted past beam stop 6.
However, the applied blanker voltage is, in fact, time dependent, i.e. the voltage is varied during the transition between the beam "on" and "off" states. This variation in blanker voltage during the transition, coupled with the beam transit for those electrons passing through aperture 5 results in undesirable motion at the target 26.
Various structures have been proposed to eliminate the problem of beam jitter. For example, U.S. Pat. No. 4,445,041 (hereinafter the '041 reference) discloses a double deflection blanker (see FIG. 1) in which the bottom halves 9 of plates 2 deflect the electrons which have passed through aperture 5. This second deflection of the electrons attempts to compensate for this time-dependent blanker voltage by creating a steady state voltage effect, thereby minimizing the beam jitter.
However, this reference describes solely an operation of the beam blanker at a 20kV beam voltage. An extension of this prior art disclosure to other beam voltages results in serious degradation in performance. Therefore, a need arises for a beam blanker which operates within the positional accuracy range at all times, for all beam voltages.